AQA AS Level Biology Paper 2, November 2021: Question 1

8 marks · Medium difficulty · Short Answer

Identify the functional group and R-group structure of fatty acids, describe the emulsion test for lipids, and compare a historical membrane model with the fluid-mosaic model.

Practise this question

Question

The question contains four parts about fatty acids and cell membranes. Question 01.1 asks to name the group represented by COOH in a fatty acid. Question 01.2 shows Figure 1, a chemical structure of a fatty acid R group containing a double bond, and asks to name the type of R group and explain the answer. Question 01.3 asks to describe the test for lipids in a liquid food sample. Question 01.4 shows Figure 2 representing a 1935 membrane model with continuous protein layers sandwiching a phospholipid bilayer, and asks for one similarity and two differences compared to the fluid-mosaic model.
Question text

01.1 The general structure of a fatty acid is RCOOH.

Name the group represented by COOH.

[1 mark]

01.2 Figure 1 shows the structure of a fatty acid R group.

Figure 1

Name the type of R group shown in Figure 1.

Explain your answer.

[2 marks]

Type of R group

Explanation

01.3 Describe how you would test for the presence of a lipid in a liquid sample of food.

[2 marks]

In 1935, scientists suggested a model for the chemical structure of a cell-surface

membrane. Figure 2 shows the membrane structure the scientists suggested.

Figure 2

01.4 Give one similarity and two differences between the membrane structure shown in

Figure 2 and the fluid-mosaic model of membrane structure.

[3 marks]

Similarity

Difference 1

Difference 2

Mark scheme

Show the mark scheme The mark scheme provides answers for questions 01.1 to 01.4. For 01.1, it accepts 'carboxyl' or 'carboxylic acid' for 1 mark. For 01.2, it awards 2 marks for stating 'unsaturated' and explaining the presence of a double bond (C=C) between carbons. For 01.3, it awards 2 marks for adding ethanol/alcohol then water and shaking, noting a white/milky emulsion. For 01.4, it awards up to 3 marks for identifying a similarity (e.g., both have a phospholipid bilayer or proteins) and two differences (e.g., 1935 model lacks channel proteins, cholesterol, glycoproteins, or glycolipids found in the fluid-mosaic model).

Question Marking Guidance Mark Comments

01.1 Carboxyl; 1 Accept carboxylic acid

01.2 Type of R group

1. Unsaturated (fatty acid/hydrocarbon); 1. Accept alkene

Explanation 2. Accept for ‘double bond’,

C=C

2. Double bond (between carbons);

01.3 1. Add ethanol/alcohol then add water and 1. Reject heating the emulsion

shake/mix test

OR 1. Accept ‘Add Sudan III and

mix’

Add ethanol/alcohol and shake/mix then

pour into/add water; 2 1. Ignore a second shake

2. White/milky (emulsion) 2. Ignore cloudy

OR 2. Reject precipitate

(emulsion) test turns white/milky; – 2. Accept (for Sudan III) topLOGY––JUNE 2021

(layer) red

01.4 (Similarity)

Accept 2 marks max if

1. is not achieved

1. Both have a phospholipid bilayer

OR

Both have fatty acid/hydrophobic tails pointing

in/face each other

OR

Both have phosphate/polar/hydrophilic heads

pointing out

OR

2., 3., 4., and 5.

accept first answer

Both have protein;

refers to 1935 model

3 max unless otherwise

(Differences)

stated

2. No channel/carrier proteins, whereas fluid 5

2. Accept for ‘no

mosaic does

channel/carrier’, no

intrinsic

OR

2. Accept only one

Protein layer outside (phospholipids), fluid mosaic

type of protein

is 'dotted';

whereas fluid

mosaic has many

3. Cholesterol is not present whereas it is present

(types)

in fluid mosaic;

4. Glycoprotein is not present whereas it is present

in fluid mosaic;

5. Glycolipid is not present whereas it is present in

fluid mosaic;

TOTAL 8

How to answer it

Biological Molecules and Cell Membranes Revision Guide

AQA AS Level Biology

What this question tests

This question assesses your knowledge of lipid chemistry (fatty acid structure and functional groups), biochemical testing procedures for lipids (the emulsion test), and your ability to compare historical scientific models of cell membranes with the accepted fluid-mosaic model.

Question 01.1

Identifying Functional Groups in Fatty Acids

✅ Correct Answer

Carboxyl (Accept: carboxylic acid)

1 mark

💡 Key Knowledge

Fatty acids consist of a long hydrocarbon tail (the R group) attached to a -COOH functional group, which defines them as organic acids.

Question 01.2

Analyzing R Group Structure

✅ Correct Answers

  • Type of R group: Unsaturated (fatty acid / hydrocarbon) / Alkene
  • Explanation: Presence of a carbon-carbon double bond ( C=C )
2 marks

❌ Common Errors

Students sometimes state the structure is "saturated" because they miscount bonds or overlook the double bond visible in Figure 1. Always scan hydrocarbon chains carefully for C=C double bonds.

Question 01.3

The Lipid Emulsion Test

✅ Correct Answers

  • Step 1: Add ethanol/alcohol, then add water and shake/mix (or add ethanol, shake, and pour into water).
  • Step 2: Observation: Solution turns white/milky (emulsion forms).
2 marks

🧠 Exam Technique & Examiner Commentary

Examiners strict on sequencing: you must add ethanol before water. Reject any answers involving heating (unlike Benedict's test, heat ruins the emulsion test). "Cloudy" is accepted for the observation, but "milky" or "white emulsion" is preferred. Reject "precipitate".

Question 01.4

Comparing Membrane Models

✅ Correct Answers (1 Similarity + 2 Differences)

  • Similarity: Both have a phospholipid bilayer / fatty acid tails pointing inward / phosphate heads pointing outward / both contain protein.
  • Difference 1: 1935 model has proteins restricted to a continuous layer on the outside surfaces, whereas the fluid-mosaic model has proteins embedded (mosaic / intrinsic / channel / carrier proteins).
  • Difference 2: 1935 model lacks cholesterol, glycoproteins, and glycolipids, whereas the fluid-mosaic model contains all three.
3 marks max

❌ Common Errors

A common pitfall is listing two similarities instead of one similarity and two differences. Read the stem carefully to ensure you satisfy all instructions. When describing differences, make sure you contrast the 1935 protein "sandwich" layer against the scattered proteins of the fluid-mosaic model.

Topics

Biology · 3.1 Biological molecules · 3.2 Cells

Question and mark scheme from the AQA AS Level Biology examination, Paper 2, November 2021. QuestionVault is an independent revision resource; questions remain the copyright of the awarding body.